The present invention is an improvement in a breathing apparatus of the type which includes a connection device for connection to the breathing organs of a user such as a helmet, a breathing mask or mouth piece, and a breathing circuit connected to the connection device. The breathing circuit comprises a carbon-dioxide absorber and an expansible gas chamber such as a breathing bag or bellows. The breathing apparatus also includes suitable controls which control the operation of the breathing circuit to provide (i) a first operating period wherein the breathing circuit is closed and a breathing gas is caused to circulate in the breathing circuit until the oxygen concentration of the circulating breathing gas is reduced to a predetermined value and (ii) a second operating period, which alternates with the first, wherein used breathing gas is released to the ambient environment and a fresh gas, the quantity of which is substantially independent of the depth of the use under the water surface, is supplied to the breathing circuit. A breathing apparatus of this kind is disclosed in U.S. Pat. No. 3,827,432 (Lundgren et al) and the disclosure of that patent is hereby incorporated by reference. Apparatus of this type possesses the advantages that very little of the gas is consumed irrespective of the diving depth and that the proportion of the oxygen used may be maintained within narrow limits. However, when the diving depth is increased, such apparatus suffer shortcomings. More specifically, when the expansible gas chamber is compressed due to the increased diving depth, a certain reduction of the floating capacity of the unit results. This loss of floating capacity is a disadvantage in many applications and an inconvenience in all applications.